Gerard Butler is standing at the window with his back to me, aiming a gun. His tree-trunk legs are spread wide. His biceps are stretching his cashmere jersey, his deltoids flexing like a pair of dolphins trying to nibble his ears. Testosterone oozes across the room.

From this angle, Butler is quite something. You feel an instant sympathy with the 1,589 members of the Facebook group Gerard Butler Is My Husband - He Just Doesn't Know It Yet. Likewise the 800-odd who signed up to Gerard Butler Can Impregnate By Touch Alone, and Gerard Butler Can Make Even Physical Deformity Sexy (presumably a reference to his role in the film Phantom of the Opera).

And then he turns round. He's not holding a gun, just squeezing his fingers round an imaginary one. And his target out there? A really nasty piece of work - got to be. After all, the hulk who has played Attila the Hun and Beowulf, and who led 300 highly oiled Spartans into battle wearing just a cape and a leather codpiece, is hardly going to waste his wrath on small fry, is he? Grendel, is it? The Persian hordes? Actually, there's one solitary and rather damp paparazzo loitering on the street below. "God damn, I hate those people," growls Butler, returning to the sofa and cracking open a Coke.

So much of Butler's fame is based on his beefcake physique - that rippling 10-pack, that big stubbly head - but the menace that bubbles up in him on screen just isn't present in real life. Not, at least, from the front. He seems, if anything, a bit lost. Half his time, he says, is spent "trying to maintain a good attitude" by way of "meditation, conversation and spending 10 minutes a day with nature".

Unlikely things torment him. Gratuitous violence on film leaves him feeling "hollow inside". "It's like when you're having a steak and you suddenly feel nauseous," he says. "You think, 'This is something dead, and I'm putting it inside me.'" Yikes. Anyway, the queasy carnivore, the reluctant action hero, is here today to promote RocknRolla, Guy Ritchie's latest geezer flick, in which he stars as One Two, a twinkle-eyed thug who gets in over his head with an old-school London mobster, played by Tom Wilkinson. Butler's giving the promo his best shot. Indeed, he's slightly overegging it. Guy, he says, is a "genius". Acting with the ensemble was fabulous. "Sometimes you pinch yourself and think, 'Fucking hell, I'm working with Thandie Newton!'"

Despite the iffy early reviews, he's keen on the film because, he says, it gives him the chance to show off his range. Although there's something a bit improbable about the idea of a Ritchie film doing that for anybody, he does have a point. "One Two is more multi-faceted than the parts I've played before," he says. "He can be tough, but he can also be clumsy, vain, self-conscious and silly." It is in many ways the perfect Butler part: a hard man with a soft centre, rough-edged but still sweet enough to accommodate the gay advances of a mate.

A winning Scottishness is crucial to Butler's appeal, particularly across the pond. The film 300 was, in the end, a kind of Greco-Braveheart. His King Leonidas gave his orders in fluent tartan, without any incongruity being felt by audiences. In the US, clearly, they associate the accent with sword-waving warriors rather than with Rab C Nesbitt. "In Scotland I'm just like a lot of other guys," he says, "but in America I'm seen as a very strong, masculine guy."

Does he think there's a difference between what makes a man a man in LA and in Glasgow? He's thoughtful. "In Glasgow, you probably grow up faster. You have to deal with more shit, more realism. In LA, it's a superficial, perfect world. In Glasgow, you're expected to be a man - but it doesn't make you the perfect man. It makes you a man who's not able to talk about his feelings."

Butler was raised by his mother after she and his father separated, the latter choosing to remain in Montreal, where Butler lived for the first two years of his life. He didn't have any further contact with his father until he was 16, when the two met up in a cafe. Butler found himself tongue-tied, and returned home to cry solidly for three hours. The pair became close, until his father died of cancer when Butler was 22.

He studied law at Glasgow University, won a traineeship in Edinburgh, but didn't much like it, preferring to hit the bottle with his pals (he was especially keen on jumping in front of moving cars and smashing bottles on his head). At 28, he quit booze for good, and then found himself fired from the law firm. So he came to London, took some odd jobs, drifted around. One day Steven Berkoff approached him in a cafe and asked if he'd ever considered acting, offering him a part in Coriolanus. Film work followed: a bit part in Mrs Brown, then Dracula, Attila, Tomb Raider: Cradle of Life and 300. These hard-men roles were interspersed with slush, such as Dear Frankie, in which he plays a dreamy father substitute to Emily Mortimer's deaf-mute son.

And now RocknRolla. Butler gives an assured performance, but it's a flashy, trashy movie that's unlikely to do much for his career. Butler has, perhaps, reached something of a plateau. For all that carefully maintained physique, he's not getting any younger and appears to be wrestling with an identity crisis. He seems genuinely stumped when I ask who his role models are, and where he sees himself in five years - hardly ambush questions, but he doesn't have answers. He says something vague about accepting his career, going on a spiritual journey, and that he's in no hurry to have children. He lives alone in New York but has been linked to a number of women, Naomi Campbell and Cameron Diaz included, but no one for long.

Is he lonely? "I definitely have my moments. It's a Scottish thing, the Celtic fog. I am prone to getting inside my head too much. I feel it quite heavily from time to time. In this job, there's more chance for that. Not all the time, just 95%. It's like: please don't leave me alone." Perhaps it is this vulnerability that gets to his fans: the hard man with the lost boy deep inside. The extraordinary outpourings on the net (other Facebook groups include Please Have Your Way With My Naked Body Gerard Butler, and Gerard I Want to Touch Your Butler) do seem to be responding to more than just the torso.

Maybe he's too introspective, he says at one point. "I grew up in a very working-class part of Glasgow," he adds. "Then I worked for a very well-to-do law firm in Edinburgh. Then I was an actor in London. Now I'm between New York and LA. I've lived in so many places it's sometimes confusing as to who I am".